Pulley assemblies connected to the crankshaft are generally provided with a damping device and enable simultaneously driving of a belt drive for accessories and damping of the torsional vibrations of the crankshaft.
In order to reduce the levels of consumption, internal-combustion engines adopt a control system referred to as start-stop, whereby an electronic control unit turns off the engine when the vehicle is stationary and re-starts the engine when the accelerator pedal is pressed.
When the internal-combustion engine is turned on, the crankshaft drives the accessories belt drive, whilst, when the engine is turned off, for example when queuing at traffic lights, the accessories belt drive is driven by a further driving machine, for example a reversible electric machine connected to the belt drive and functioning either as alternator or as electric motor in response to a control signal of the electronic control unit.
Pulley assemblies are known provided with a hub, designed to be connected to a crankshaft of an internal-combustion engine, a seismic mass connected to the hub by an element made of elastomeric material so as to define a device for damping the torsional vibrations, and a pulley, rotatably mounted on the hub via a bearing. Known pulley assemblies further comprise an armature connected in an angularly fixed and axially mobile way to the hub and actuated via an electromagnet controlled by the electronic control unit.
In use, the armature is pressed axially against a wall of the pulley via an elastic element and, consequently, provides a friction connection that is angularly fixed between the hub and the pulley.
When the internal-combustion engine is turned off, the electronic control unit energises the electromagnet, and the mobile armature recedes against the action of the spring. Consequently, the pulley is de-coupled from the hub, and the belt drive can be actuated by the electric machine whilst the crankshaft remains angularly fixed.
The ever increasing value of the torque absorbed by the accessories calls for values of axial thrust which can be obtained only using springs of increasingly larger dimensions. Said springs moreover require a high current consumption by the electromagnet, and consequently part of the advantages of the start-stop system are nullified.